In recent years, the need arises for a shaft such as the main spindle of a working machine to exhibit higher rotation and higher accuracy. There is a growing demand for a rolling element made of ceramics that is light and highly tough as a rolling element of a rolling bearing that supports such a shaft. A rolling element made of ceramics has high insulation. Therefore, the potential of a rolling element of a bearing for use in power generators and motors is high. There is also the potential for use in transportation machines such as aircrafts and automobiles by virtue of its lightweight property as compared to a steel ball that is the general rolling element of a bearing.
By the demand of high reliability in rolling bearings, 100% inspection is required in the manufacturing of rolling elements made of ceramics to detect any defect in the material and work. The size of the defect to be detected is generally greater than or equal to 20 μm, depending upon the material strength and the like.
A scratch on the surface of the rolling element, a defect in the inclusion and the like may be detected by an appearance inspection through visual confirmation, the method of inspecting the reflected state of light from the surface of a rolling element using an optical system, and the like. Since the time required for such inspection is short according to these methods, they can be employed in mass production processes.
However, in rolling elements made of ceramics, the durability may be degraded even in the case where there is a defect, not only at the surface, but also inside. Therefore, the inspection for a defect must be performed for the entire interior in addition to the surface of the rolling element.
As a method of detecting a defect present inside a component made of ceramics, there is proposed a method including the steps of applying integration processing on an image obtained by X-ray transmission, and differentiating the integration-processed image to detect an internal defect (for example, refer to Japanese Patent Laying-Open No. 8-146137 (PTD 1)). However, this method requires a long period of time for image pickup since pictures for the integration processing must be taken. It is therefore difficult to employ this method in the inspection for the mass production of rolling elements.
There is also known a method of evaluating the porosity distribution of silicon nitride sintered body using X-ray CT (Computed Tomography) (for example, refer to Japanese Patent Laying-Open No. 2001-201465 (PTD 2)). According to this method, a pore of approximately 10 μm can be detected. However, this method also requires a significant period of time for image pickup. Therefore, it is difficult to employ this method in the inspection method for mass production of rolling elements.
There is additionally known a method of measuring the porosity distribution in a ceramic structure by driving an X-ray source and a detector helically in a scanning operation using medical X-ray CT (for example, refer to Japanese Patent Laying-Open No. 2005-283547 (PTD 3)). This method allows a ceramic structure to be inspected in a relatively short time. However, the aforementioned helical scanning operation by an X-ray source and detector requires the inspection to be carried out based on images corresponding to a predetermined distance. The entirety of the subject of inspection cannot be inspected without omission. Therefore, it cannot be said that this method is appropriate for a rolling element made of ceramics that requires high reliability.